When Poland regained independence in November 1918, Jędrzejewicz began work at the Polish Army's Section II (Oddział II, or Intelligence).

On 24 April 1920, Jędrzejewicz, now a captain, signed a military convention with Ukraine's AtamanSemen Petlura which paved the way for the Polish Army's 1920 Kiev Expedition. Next he served as Section II chief successively to Generals Kazimierz Sosnkowski and Gustaw Zygadłowicz. In September–November 1920, as a major, he was the Polish Army's liaison officer to allied Belarusian forces.

Returning to Poland, he served as director of the Foreign Ministry's Consular Department (1928-1933) and as Treasury Vice Minister (1933-1934). On 22 January 1934, he was appointed Minister of Religious Denominations and Public Education in the government of his brother, PremierJanusz Jędrzejewicz (1885-1951), serving on as well in the premierships of Leon Kozłowski and Walery Sławek. He introduced educational reforms that sparked controversy in Poland but won international approval and emulation.

After Marshal Józef Piłsudski died (1935), Jędrzejewicz held no more ministerial offices.

Jędrzejewicz published some 300 scholarly papers in history and several major books, including Poland in the British Parliament, 1939-45 and a two-volume Kronika życia Józefa Piłsudskiego (Chronicle of the Life of Józef Piłsudski). His English-language publications also included Piłsudski: a Life for Poland, New York, Hippocrene Books, 1982.